Static control for step voltage regulator

ABSTRACT

A static control for a step voltage regulator derives a lower bandwidth limit potential at the wiper of a bandwidth setting potentiometer connected across a reference voltage source and an upper bandwidth limit potential at the output of an inverter operational amplifier having inputs coupled to the wiper and to the reference voltage source. First and second comparators respectively compare the upper and lower bandwidth limit potentials to a sample voltage proportional to the regulator output potential. A timer includes a pulse generator for deriving pulses at power line frequency, a digital counter incremented by the pulses, a digital-to-analog network for deriving a unidirectional voltage proportional to the count stored in the counter, and a comparator for providing a timer output when the unidirectional voltage reaches a reference potential. A logic gate normally disables the counter and in response to the output of either comparator to enable the counter and thus start the timer. First and second triggerable semiconductor switches respectively energize the raise and lower windings of the tap changer motor. A latch has cross-connected first and second logic gates respectively controlled by the first and second comparators and each of which receives the timer output and firing pulses from a high frequency oscillator and is enabled when the outputs from the corresponding comparator, the timer, and the oscillator occur simultaneously. Firing circuits controlled by the first and second logic gates of the latch trigger the second and first semiconductor switches to thereby energize the raise and lower windings of the tap changer motor.

United States Patent Gilmore Oct. 14, 1975 STATIC CONTROL FOR STEP VOLTAGE width setting potentiometer connected across a refer- REGULATOR ence voltage source and an upper bandwidth limit potential at the out ut of an inverter 0 erational am li- [75] Inventor: 333 Gilmore wauwatosa fier having inputs coupled to the wip r and to the aference voltage source. First and second comparators [73] Assignee: Allis-Chalmers Corporation, respectively compare the upper and lower bandwidth Milwaukee, Wis. limit potentials to a sample voltage proportional to the re ulator out ut otential. A timer includes a ulse [22] Flled: 1974 geierator for derising pulses at power line frequ ncy, [21] Appl. N0.: 520,036 a digital counter incremented by the pulses, a digitalto-analog network for deriving a unidirectional voltage Us. CL 323/435 318/280 323/45 proportional to the count stored in the counter, and a [51] Int Cl 2 llllllllll U GOSF U20 comparator for provldmg a t1mer output when the um- [58] Head 0'1 351:1. i /35325554, 6, 43.5 s, e voltag; 3 f gq A ogic ga e norma y Isa es e coun er an in re- 323/45 5l 54 318/280 290 sponse to the output of either comparator to enable the counter and thus start the timer. First and second [56] Referencgs cued triggerable semiconductor switches respectively ener- UNITED STATES PATENTS gize the raise and lower windings of the tap changer 3,237,091 2/ l966 Attewell 323/435 S motor, A latch has cross-connected first and second 3 1 1967 Mccabe et 323/45 X logic gates respectively controlled by the first and sec- 3,585,492 6/1971 McCabe 323/45 X 0nd comparators and each of which receives the timer et 43 17 52 output and firing pulses from a high frequency oscilla- 317251771 4 1973 Gilmore... .1333/435 s X is enabled when from the 9 3,818,321 6/1974 Willner et al 323/435 5 SPOndmg comparator, the and the Osclllato' Primary Examiner-Gerald Goldberg Attorney, Agent, or FirmLee H. Kaiser [57] ABSTRACT A static control for a step voltage regulator derives a lower bandwidth limit potential at the wiper of a band- SOURCE (310 was LILMEB 1.

occur simultaneously. Firing circuits controlled by the first and second logic gates of the latch trigger the second and first semiconductor switches to thereby energize the raise and lower windings of the tap changer motor.

V 15 Claims, Z 'Drawing Figures l l l IJBACJERLEG US. Patent Oct. 14, 1975 fi ummiumow STATIC CONTROL FOR STEP VOLTAGE REGULATOR This invention relates to a control for a step voltage regulator and in particular to a static system for controlling a reversible motor which actuates the movable tap changer contacts of a step voltage regulator to maintain the regulator output voltage within upper and lower bandwidth limits.

BACKGROUND OF THE INVENTION A step voltage regulator is an autotransformer provided with load ratio control equipment for regulating the voltage on the feeder or bus to which it is connected. The step voltage regulator may have a 100 percent exciting winding in shunt with the line on the source side and normally maintains the voltage on the load side within a desired voltage bandwidth by a percent tapped buck/boost winding in series with the line. The 10 percent series winding has taps connected to stationary contacts of a tap changer switch having a pair of rotatable contacts driven by a reversible motor and usually provides the ability to change the effective turns ratio from input to output plus or minus 10 percent in 32 steps of five-eighths percent voltage.

Static controls for step voltage regulators are known of the type disclosed in my US. Pat. No. 3,725,771 having the same assignee as this invention which sense the voltage on the load side and operate raise and lower electromagnetic relays when the load voltage is beyond the lower and upper bandwidth limits to complete energizing circuits to corresponding windings of the tap changer motor to again bring the voltage within the bandwidth. Although such controls have been found satisfactory, the electromagnetic relays make the cost of the circuits relatively expensive. Moreover, such electromagnetic relays inherently require adjustment and are subject to mechanical failure.

OBJECTS OF THE INVENTION It is an object of the invention to provide an improved static control for a step voltage regulator which eliminates the expensive electromagnetic relays of known controls and utilizes semiconductor components in the motor energizing circuit that do not require adjustment and are inherently trouble-free.

A further object of the invention is to provide such an improved static control for a step voltage regulator which is more economical to manufacture and more reliable in operation than known controls.

Another object of the invention is to provide such a static control having an improved and novel circuit for sensing when the load voltage is beyond the upper and lower bandwidth limits.

Still another object is to provide such a step voltage regulator static control which has a digital timer that uses the line frequency as a time base and is more accurate in operation and lower in cost than timers of known controls.

SUMMARY OF THE INVENTION A step voltage regulator control has sensing means for deriving a DC. sample voltage which is propor tional to the regulator output voltage, a DC. reference voltage source, a bandwidth setting potentiometer connected in a circuit across the reference voltage source for deriving a lower limit reference potential at the potentiometer wiper which is a preselected fraction of the reference voltage, and a unity gain inverter amplifier receiving the reference voltage and lower limit reference potential as inputs for deriving an upper limit reference potential equal to the sum of the reference potential plus the difference between the reference voltage and the lower limit reference potential. First and second voltage comparators compare the sample voltage to the upper limit reference potential and to the lower limit reference potential respectively.

A timer preferably includes a pulse generator for deriving a train of pulses at power line frequency, a digital counter incremented by the pulses, a digital-to-analog network for deriving a unidirectional timing" voltage proportional to the count stored in the counter, and a comparator for providing a timer output when the timing voltage rises to the level of a predetermined reference potential. Logic gate means normally disable the counter and are responsive to the output of the first voltage comparator or to the output of the second voltage comparator to enable the counter and thus start the timer.

First and second semiconductor switching means respectively energize the raise and lower windings of the tap changer motor. A latch comprises cross-connected first and second logic gates which are respectively controlled by the first and second voltage comparators to prevent simultaneous conduction by the semiconductor switching means. Each logic gate of the latch also receives the timer output and firing pulses from a high frequency oscillator and is enabled when outputs from the corresponding voltage comparator, the timer, and the oscillator occur simultaneously. Firing circuits controlled by the enabling of the latch first and second logic gates trigger the second and first semiconductor switching means respectively to thereby energize the tap changer motor raise and lower windings.

DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram of a static control for a step voltage regulator embodying the invention; and

FIG. 2 is a schematic circuit diagram of the digital-toanalog converter ladder network of the timer of the control shown in FIG. 1.

DETAILED DESCRIPTION Referring to the drawings, a known step voltage regulator has an exciting winding SH connected across its S and SL terminals which are adapted to be connected to the conductors 10 and 11 of an alternating current power line to be regulated and a series winding SE connected in series with the line and provided with taps connected to the stationary contacts 1-8 of a tap changer switch which are arranged in a circle. One side of exciting winding SH is connected to the respective ends of series winding SE by an automatic, mechanically operated reversing switch RS which reverses the polarity of series winding SE and thereby doubles the range of the tap changer. Two rotatable controls 14 and 15 of the tap changer sequentially engage the stationary contacts 1-8 and N and are connected through slip rings to the opposite ends of a midtapped preven- 3 tive autotransformer PA whose midtap is connected to the regulator terminaIL which is adapted to be con nected to the load. -The rotatable contacts 14 and are actuated by'a reversible motor M in a first direction to raise the output voltage when the RAISE winding of 5- the motor is energized and in the opposite direction'to lower the output voltage of the step regulator when the LOWER winding of the motor is energized. The step voltage regulator may include a potential transformerformer TRl having a plurality of taps inits primary to permit the selection of any-load voltage level between 105 and 134 volts in one volt steps by adjustment of a voltage setting rotary coarse" switch SW2 and a voltage setting fine or vernier switch SW3.

SENSING STAGE AND BANDWIDTl-I CIRCUIT The secondary winding S2 of potential sensing transformer TR-l is coupled 'to a full wave bridge rectifier comprising diodes D1-D4 of the calibration section CALIB of the voltage sensing stage of the control. The rectified voltage is coupled through a calibrating rheostat RHl and a ripple-removing resistance capacitance filter R2, CI, R3 and derives a unidirectional sample voltage across'a capacitor C2 at point TPlproportional to the power line'voltage at the output of the regulator.

"A reference voltage v, is developed at a point TP3 across' a'Zener'diode Z3 of the sensing stage reference section REF. Zener diode Z3 is connected in series with a resistance R5 between a regulated power supply +1 2V and ground, and reference voltage v, is applied across the series arrangement of a resistance R6, a bandwidth setting rheostat RH2, and a resistance R8 to ground. The sensing stage is initially calibrated by setting sample voltage v at point TPl equal to reference voltage v, at point TP3 by adjusting rheostat RHl while a nominal potential of 120 volts is applied across the primary of sensing transformer TRl. The sensing section 'SENSE compares changes in sample voltage v with the reference voltage v, and the amount of change in v, to be allowed before a tap change occurs is set by adjustment of rheostat RH2. The sensing stage derives an upper limit reference voltage and a lower limit reference voltage which are respectively more positive and more negative than the reference voltage V, by a preselected portion of reference voltage v,. The preselected portion of v, is established by the setting of rheostat RH2. The lower limit reference voltage v is derived at the wiper of rheostat RH2and is more negative than reference voltage V,. The upper limit reference voltage is derived at the output of a unity gain operational am plifier inverter 5A which in effect sums the reference voltage v, plus the difference between v and v, (i.e., v, v and inverts the polarity of the resultant voltage. Reference voltage v, is coupled to the noninverting input of inverter 5A and the lower limit reference voltage v, is applied to its inverting input so that the output of inverter 5A is more positive than v by (v, v,)."

The bandwidth sensing section SENSE comprise'sop'g erational amplifiers 4A and 48 connected as comparators. Amplifier 4A senses when the'power line voltage detects when power line voltage drops below the lower bandwidth limit. The sample voltage v at point TPl proportional to the power line voltage is coupled to the inverting inputs of both amplifiers 4A and 4B. The lower limit reference voltage v, derived at the wiper of bandwidth-adjusting rheostat RH2 is'applied to the noninverting input of comparator 4B. The upper limit reference voltage derived'at the output of inverter 5A is coupled to the noninverting input of comparator 4A. Initial calibration is accomplished by applying volts across the primary of sensing transformer TRl and then adjusting calibrating rheostat Rl-Il until the sample potential v at point TPl is equal to the reference voltage v, at TP3. This is the only calibration required for the control. I g

Since the inverting input of 4A sees the sample voltage v at TPl and its noninverting input receives the upper limit reference output of inverter 5A (which is higher than the reference voltage v', by the calibrated portion of v, set on bandwidth adjusting potentiometer RH2),'t he output of comparator 4A is logic 1 at null (i.e., initially) and switches to logical zero when the line voltage exceeds the upper bandwidth limit. Similarly, since the inverting input of 4B sees the sample voltage v at TH and its noninverting input receives the lower limit reference voltage v, from the slider of rheostat RH2, the output of 4B is logic 0 at null and switches to logic 1 if the line voltage drops belowthe lower bandwidth limit.

LOGIC When the sensing stage detects that the power line voltage is beyond one of the bandwidthlimits, the output of comparator 4A or 4B actuates the LOGIC circult to: (a) start the TIMER, and (b) select which gatetriggered silicon AC switch TRIAC 1 or TRIAC 2 is to be triggered into conduction to energize the motor after the TIMER times out.

The output of comparator 4B is logic Oat null and is coupled to one input of a NORv gate 2B. The output of comparator 4A is logic 1 at null-and'is coupled through a NOT gate, or inverter 2A to another input of NOR gate 28. Consequently, both inputs of NORgates 2B are logic 0 atnull and it output is logic 1 which is coupled to the reset (R) input of a COUNTER of the TIMER, thereby disabling the timer; When the power linevoltage exceeds one of the bandwidth limits, comparator 4A or 4B switchesstates to thereby change the output gate 28 to logic 0 to start the "TIMER.

A pair of NAND gates 1A and lB'cornprises a latch which controls which silicon AC switch, or bidirectional controlled rectifier TRIAC l or TRIAC 2 is to be triggered on. The output of NOT gate-2A, which is logic 0 at null, is coupled to one in-put'of gate 1A, and the logic 0 output of gate 48 at mill is coupled to one input of gate 1B. The logic 1 signal from the TIMER when it times out is coupled to one input of each of gates 1A and 1B. A third input to each gate 1A and 1B is from a 10 Khz oscillator OSC to provide firing pulses for the silicon AC switches as described hereinafter. The output of gate 1A is connected to one input of gate 13, and the 'output of gate 18 is connected to an input of gate 1A to provide a latch configuration which assures that only one gate 1A or 18 at a time can be enabled and thereby preventing the firing of both silicon ACswitch'es simultaneously.

When the load voltage exceeds one bandwidth limit,

for example, the upper limit, thesensing stage output TIMER The TIMER is of the digital type and uses the power line frequency as a time base. A pulse generator PG that derives 60 Hz timing pulses from the power line includes an amplifier comprising transistors T5 and T6 with positive feedback provided by a resistance R39. One side of a midtapped tertiary winding S3 of sensing transformer TRl is coupled through two series resistances R41 and R40 to the base of NPN transistor T6 so' that it is turned on and'off by the half cycles of power line voltage. Turning on T6 lowers the voltage on the base of normally conducting transistor T5 and turns it off to generate a positive-going pulse at the collector of T5 which is fed back through resistance R39 to the' base of NPN transistor T6. Resistance R40 and capacitor C constitute a filter which prevents noise from creating false timing pulses. Diode D12 protects transistor T6 from reverse bias.

The 60 Hz pulses from pulse generator PG are coupled through a NOR gate 2C to a digital pulse COUNTER which counts positive-going transitions from NOR gate 2C. Logic 0 from gate 2B is coupled to the reset input (R) to the counter to start the TIMER, and each, negative-going transition of the collector of T5 enables gate 2C so that it provides a logic 1 timing pulse to the COUNTER. The .COUNTER may be a known multi-stage binary counter wherein each stage is a master-slave flip-flop (not shown) and the counter is advanced one count on the negative-going transition of each input pulse, such as the commercially available CMOS l4-stage counter having the type designation SCL4020. The binary coded decimal output of the COUNTER is preferably changed to a stepped analog voltage proportioned to the stored count by an R-2R resistor ladder network RNl shown in FIG. 2. The output from resistor ladder network RNl is a stepped ramp rising with time wherein each step may be 43 millivolts high and occurs approximately every 2 seconds so that the instantaneous voltage of the RNl network output is an analog of the stored count. the output of network RNl is coupled to the noninverting output of a comparator operational amplifier 5B whose inverting input receives a reference voltage from a time delay adjusting rheostat RH3. Rheostat R113 is connected in series with two resistances between the power supply +12V and ground and is adjusted to provide the desired time delay from TIMER and at null results in logic 0 output from comparator 5B. When the analog output voltage from ladder network RNl is equal to the reference voltage from rheostat R113, the output of comparator 5B switches to logic 1. The logic 1 output from comparator 5B is applied to one input to NOR gate 2C to disable it and thus block pulses from the 60 Hz pulse generator to the COUNTER and is alsoapplied to one input of each gate 1A and lBof the logic latch to enable the appropriate gate and thus allow the corresponding silicon AC switch TRIAC l or TRIAC 2 to be fired.

An oscillator OSC supplies a high frequency train of 10 Khz pulses to one input of each gate 1A and 18 to provide firing pulses for the corresponding silicon AC switches TRIAC I or TRIAC 2 when gate 1A or 1B is enabled. The high frequency oscillator OSC is known and comprises two integrated circuits 1C and 1D and preferably is of the type CD4012AF described on page 354 of RCA DATA BOOK SSD-203A. The 10 Khz pulses are inverted by NOT gate 2D and applied to one input of each gate 1A and 1B.

SILICON SWITCH FIRING CIRCUITS The firing circuits TF for the silicon AC switches, or bidirectional controlled rectifiers TRIAC 1 and TRIAC '2 are identical and only the one for TRIAC 1 will be described. At null, both gates 1A and 13 have a logic 1 output. When the power line voltage drops below the lower bandwidth limit, sensing gate 48 provides a logic 1 output to NAND gate 18 and also to gate 2B to start the TIMER. When the TIMER times out and gate 58 thereof applies logic 1 to one input of each of gates 1A and 1B, all four inputs to gate 1B will be logic 1 when a vl0 Khz pulse occurs, and gate lB will switch to a logic 0 output for about 14 microseconds (the width of a 10 Khz pulse) at a 10 Khz pulse rate. The 14 microsecond logic 0 pulse from gate 1B is coupled through a resistance R20 to the base of a PNP transistor T4 and turns it on. A capacitor connected in shunt to resistance R20 speeds up the switching of T4. Flow of current through collector resistance R26 of T4 develops a voltage drop which forward biases and turns on an NPN transistor T3. Conduction by T3 applies the +12 volt source through the primary of a pulse transformer TR2 in series with the collector-emitter circuit of T3 to ground, thereby inducing a pulse in the secondary of TR2 which is applied to the gate of silicon AC switch TRIAC 1 and turns it on. Conduction by TRIAC 1 energizes the RAISE winding of motor M which rotates tap changer contacts 14 and 15 in a direction to raise the power line voltage applied to the load. The gating pulses from the secondary of pulse transformer TR2 are applied at a frequency of 10 Khz to assure that TRIAC 1 will remain in conduction and operate motor M.

A resistance R24 in series with a capacitor C4 is connected across silicon AC switch TRIAC 1 to prevent faulty firing as a result of noise that might be induced in the leads to the motor M. A saturating choke Ll may be connected in the output lead from the silicon AC switch TRIAC 1 to limit possible current surges during initial conduction and prevents failure resulting from di/dt and dv/dt effects. A metal oxide variable resistance transient suppression device RVl such as the type commercially available from General Electric Company under the designation SC 241 E may be connected between the common return lead to the silicon switches and ground to clip high voltage transients to a safe level.

POWER SUPPLY The power supply for the control includes midtapped sensing transformer tertiary winding S3 whose ends are coupled to a full wave bridge rectifier comprising diodes D5-D8. The rectified pulses from opposite ends of bridge rectifier D5-D8 are coupled through surge resistors R7 and R29 respectively to filter capacitors C8 and C9 to provide a plus and minus supply. The voltages across filter capacitors C8 and C9 are applied through resistances R37 and R38 across Zener diodes Z1 and Z2 to provide regulated :12 volt to the sensing, logic and timing stages.

While only a single embodiment of my invention has been illustrated and described, many modifications and variations thereof will be readily apparent to those skilled in the art, and consequently, it should be understood that I do not intend to be limited to the particular embodiment shown and described.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a control for a step voltage regulator adapted to raise and lower the regulator output voltage when raise and lower windings of a tap changer reversible electric motor are respectively energized, in combination,

sensing means for deriving a sample voltage which is a function of the regulator output voltage,

a predetermined reference voltage source,

means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than the reference voltage by a preselected fraction of said reference voltage and including bandwidth setting means for adjusting said preselected fraction,

first and second comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential,

a timer adapted to provide an output a preselected time interval after it is started,

first and second triggerable semiconductor switching means for respectively energizing said raise and lower motor windings,

means for starting said timer in response to operation of said first comparator means or to operation of said comparator means, and

first firing means for triggering said first semiconductor switching means in response to the simultaneous occurrence outputs from said timer and from said second comparator means, and

second firing means for triggering said second semiconductor switching means in response to the simultaneous occurrence of outputs from said timer and from said first comparator means.

2. In a step regulator control in accordance with claim I wherein said bandwidth setting means includes a bandwidth setting potentiometer having its resistance element connected in a circuit across said reference voltage source and said lower limit reference potential is derived at the wiper of said potentiometer.

3. In a step regulator control in accordance with claim 2 wherein said means for deriving said upper limit reference potential includes a unity gain inverter amplifier having inputs coupled respectively to said ref erence voltage source and to said wiper of said bandwidth setting potentiometer.

4. In a step regulator control in accordance with claim 3 and including calibrating means for making said sample voltage equal to said reference voltage when the regulator output voltage is of the desired magnitude.

5. In a step regulator control in accordance with claim 4 wherein said timer includes 1 a pulse generator for deriving a train of pulses at the frequency of the power line being regulated, a normally disabled digital counter receiving said train of pulses, I

digital-to-analog converting means for deriving a unidirectional timing voltage whose magnitude is proportional to the count stored in said counter, and

comparator means for providing said timer output when said unidirectional timing voltage rises to the level of a predetermined reference potential, and wherein said means for starting said timer enables said digital counter.

6. In a step regulator control in accordance with claim 1 and including a latch between said comparator means and said firing means including cross-connected first and second logic gates which are respectively controlled by said first and second comparator means and whose outputs respectively control said second and first firing means.

7. In a step regulator control in accordance with claim 6 and including an oscillator for providing a train of high frequency triggering pulses for said semiconductor switching means, and wherein each of said first and second logic gates of said latch receive said timer output and said high frequency triggering pulses as inputs and is enabled when the timer, oscillator, and associated comparator means inputs thereto occur simultaneously.

8. In a step regulator control in accordance with claim 1 wherein said timer includes a pulse generator for deriving a train of pulses at thefrequency of the power line being regulated,

a normally disabled digital counter receiving said train of pulses,

digital-to-analog converting means for deriving a unidirectional timing voltage whose magnitude is porportional to the count stored in said counter, and comparator means for providing said timer output when said unidirectional timing voltage rises to the level of a predetermined reference potential, and wherein said means for starting said timer enables said digital counter. ii

9. In a control fora step voltage regulator having means to raise and lower the regulator output voltage when raise and lower windings of a reversible motor are respectively energized,

sensing means for deriving a sample voltage which is a function of the regulator output voltage,

a predetermined reference voltage source,

means including a bandwidth setting potentiometer having its resistance element connected in a circuit across said reference voltage source for deriving a lower limit reference potential at the wiper of said potentiometer which is a preselected fraction of said reference voltage,

lower limit comparator means for comparing said sample voltage to said lower limit reference potential,

unity gain inverter means having inputs coupled to I said potentiometer wiper and to said reference voltage source for deriving an upper limit reference potential which is greater than said reference voltage by the difference between said reference voltage and said lower limit reference potential,

upper limit comparator means for comparing said sample voltage to said upper limit reference potential,

first and second semiconductor switching means for respectively energizing said raise and lower motor windings,

first firing means controlled by said upper limit comparator means for triggering said second semiconductor switching means, and

second firing means controlled by said lower limit comparator means for triggering said first semiconductor switching means.

10. In a control for a step voltage regulator having means to raise and lower the regulator output voltage when raise and lower windings of a reversible electric motor are respectively energized,

sensing means for deriving a sample voltage which is proportional to the regulator output voltage,

a predetermined reference voltage source,

means including a bandwidth setting potentiometer for deriving a lower limit reference potential which is a preselected fraction, of said reference voltage,

means coupled to said reference voltage source and to the wiper of said bandwidth setting potentiometer for deriving an upper limit reference potential whose magnitude is equal to the sum of said reference voltage plus the difference between said reference voltage and said lower limit reference potential,

first and second voltage comparator means for comparing said sample voltage respectively to said upper limitreference potential and to said lower limit reference potential,

a timer adapted to be triggered into operation and to provide an output a preselected time interval after it is started,

first and second triggerable semiconductor switching means for respectively energizing said raise and said lower motor windings,

means for starting said timer in response to operation of said first voltage comparator means or to operation of said second voltage comparator means,

first firing means for triggering said first semiconduc tor switching means in response to the simultaneous occurrence of outputs from said timer and from said second voltage comparator means, and

second firing means for triggering said second semiconductor switching means in response to the simultaneous occurrence of outputs from said timer and from said first voltage comparator means.

11. In a control for a step voltage regulator having means to raise and means to lower the regulator output voltage, in combination,

sensing means for deriving a DC. sample voltage which is proportional to the regulator output voltage,

a DC. reference voltage source,

means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than the reference voltage by a fraction of said reference voltage and including bandwidth setting means for selectively adjusting said fraction,

first and second voltage comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential,

pulse generator means for deriving a train of pulses at the frequency of the power line being regulated,

a digital counter receiving said train of pulses from said pulse generator,

5 means normally disabling said counter and being operated to enable said counter in response to an output from said first voltage comparator means or to an output from said second comparator means,

digital-to-analog converting means for deriving a DC. voltage whose magnitude is a function of the count stored in said counter,

comparator means for providing a timer output when said DC. voltage from said converting means rises to the level of a time delay establishing reference potential,

means responsive to simultaneous outputs from said timer and said first voltage comparator means for actuating said means to lower the regulator output voltage, and

means responsive to simultaneous outputs from said timer and said second voltage comparator means for actuating said means to raise the regulator out put voltage.

12. In a control for a step voltage regulator having means to raise and lower the regulator output voltage when raise and lower windings of a reversible electric motor are respectively energized,

sensing means for deriving a DC. sample voltage which is a function of the regulator output voltage,

a DC. reference voltage source,

means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than the reference voltage by a preselected fraction of said reference voltage and including bandwidth setting means for adjusting said preselected fraction,

first and second voltage comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential,

a pulse generator for deriving a train of pulses at the frequency of the power line being regulated,

a digital source receiving said train of pulses from said pulse generator,

means normally disabling said counter and being operated to enable said counter in response to an output from said first voltage comparator means or to an output from said voltage comparator means,

digital-to-analog converting means for deriving a DC. voltage whose magnitude is a function of the count stored in said counter,

comparator means for providing a timer output when said DC. voltage from said converting means rises to the level of a time delay establishing reference potential,

first and second triggerable semiconductor switching means for respectively energizing said raise and lower motor windings,

first firing circuit means for triggering said first semiconductor switching means in response to the simultaneous occurrence of an output from said second voltage comparator means and said timer output, and

second firing circuit means for triggering said second semiconductor switching means in response to the simultaneous occurrence of said timer output and an output from said first voltage comparator means.

13. In a control for a step voltage regulator adapted to raise and lower the regulator output voltage when raise and lower windings of a tap changer reversible electric motor are respectively energized, in combination,

sensing means for deriving a DC. sample voltage which is a function of the regulator output voltage,

a DC. reference voltage source,

means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than said reference voltage by a preselected portion of said reference voltage andincluding bandwidth setting means for. adjusting said preselected portion, g

first and second voltage comparator means for comparing said sample voltage to said upper limit reference potential and to said lower limit reference potential respectively,

a timer,

means for starting said timer in response to operation of said first or of said second comparator means,

a high frequency oscillator for providing a train of triggering pulses, first and second semiconductor switching means for respectively energizing said raise and lower motor windings, I

a latch comprising first and second logic gates which are respectively controlled by said first and second comparator means and are cross-connected so that only one logic gate at a time can be enabled, said first and second logic gates each receiving the output of said timer and the triggering pulses from oscillator as inputs and being respectively enabled when the timer, oscillator, and associated comparator means inputs thereto occur simultaneously,

and

first and second firing means responsive to the enabling of said first and of said second logic gates of said latch respectively for triggering said first and second semiconductor switching means.

14. In a control in accordance with claim 13 wherein said bandwidth setting means includes a bandwidth setting potentiometer having its resistance element connected in a circuit across said reference voltage source so that said lower limit reference potential is derived at the potentiometer wiper, and said means for deriving an upper limit reference potential includes a unity gain inverter operational amplifier having its inputs respectively coupled to said reference voltage source and to said wiper of said bandwidth setting potentiometer.

15. In a control in accordance with claim 13 wherein said timer includes a pulse generator for deriving a train of pulses at the frequency of the power line being regulated, a digital counter receiving said train of pulses, digital-to-analog converting means for deriving a DC. timing voltage whose magnitude is proportional to the count stored in said .counter, and comparator means for providing a timer output when said D.C. timing voltage rises to the level of a predetermined time delay establishing reference potential, said means for starting said timer normally disabling said digitalcounter and being adapted to enable said digital counter in response to operation of said first or of said second voltage comparator means. 

1. In a control for a step voltage regulator adapted to raise and lower the regulator output voltage when raise and lower windings of a tap changer reversible electric motor are respectively energized, in combination, sensing means for deriving a sample voltage which is a function of the regulator output voltage, a predetermined reference voltage source, means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than the reference voltage by a preselected fraction of said reference voltage and including bandwidth setting means for adjusting said preselected fraction, first and second comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential, a timer adapted to provide an output a preselected time interval after it is started, first and second triggerable semiconductor switching means for respectively energizing said raise and lower motor windings, means for starting said timer in response to operation of said first comparator means or to operation of said comparator means, and first firing means for triggering said first semiconductor switching means in response to the simultaneous occurrence outputs from said timer and from said second comparator means, and second firing means for triggering said second semiconductor switching means in response to the simultaneous occurrence of outputs from said timer and from said first comparator means.
 2. In a step regulator control in accordance with claim 1 wherein said bandwidth setting means includes a bandwidth setting potentiometer having its resistance element connected in a circuit across said reference voltage source and said lower limit reference potential is derived at the wiper of said potentiometer.
 3. In a step regulator control in accordance with claim 2 wherein said means for deriving said upper limit reference potential includes a unity gain inverter amplifier having inputs coupled respectively to said reference voltage source and to said wiper of said bandwidth setting potentiometer.
 4. In a step regulator control in accordance with claim 3 and including calibrating means for making said sample voltage equal to said reference voltage when the regulator output voltage is of the desired magnitude.
 5. In a step regulator control in accordance with claim 4 wherein said timer includes a pulse generator for deriving a train of pulses at the frequency of the power line being regulated, a normally disabled digital counter receiving said train of pulses, digital-to-analog converting means for deriving a unidirectional timing voltage whose magnitude is proportional to the count stored in said counter, and comparator means for providing said timer output when said unidirectional timing voltage rises to the level of a predetermined reference potential, and wherein said means for starting said timer enables said digital counter.
 6. In a step regulator control in accordance with claim 1 and including a latch between said comparator means and said firing means including cross-connected first and second logic gates which are respectively controlled by said first and second comparator means and whose outputs respectively control said second and first firing means.
 7. In a step regulator control in accordance with claim 6 and including an oscillator for providing a train of high frequency triggering pulses for said semiconductor switching means, and wherein each of said first and second logic gates of said latch receive said timer output and said high frequency triggering pulses as inputs and is enabled when the timer, oscillator, and associated comparator means inputs thereto occur simUltaneously.
 8. In a step regulator control in accordance with claim 1 wherein said timer includes a pulse generator for deriving a train of pulses at the frequency of the power line being regulated, a normally disabled digital counter receiving said train of pulses, digital-to-analog converting means for deriving a unidirectional timing voltage whose magnitude is porportional to the count stored in said counter, and comparator means for providing said timer output when said unidirectional timing voltage rises to the level of a predetermined reference potential, and wherein said means for starting said timer enables said digital counter.
 9. In a control for a step voltage regulator having means to raise and lower the regulator output voltage when raise and lower windings of a reversible motor are respectively energized, sensing means for deriving a sample voltage which is a function of the regulator output voltage, a predetermined reference voltage source, means including a bandwidth setting potentiometer having its resistance element connected in a circuit across said reference voltage source for deriving a lower limit reference potential at the wiper of said potentiometer which is a preselected fraction of said reference voltage, lower limit comparator means for comparing said sample voltage to said lower limit reference potential, unity gain inverter means having inputs coupled to said potentiometer wiper and to said reference voltage source for deriving an upper limit reference potential which is greater than said reference voltage by the difference between said reference voltage and said lower limit reference potential, upper limit comparator means for comparing said sample voltage to said upper limit reference potential, first and second semiconductor switching means for respectively energizing said raise and lower motor windings, first firing means controlled by said upper limit comparator means for triggering said second semiconductor switching means, and second firing means controlled by said lower limit comparator means for triggering said first semiconductor switching means.
 10. In a control for a step voltage regulator having means to raise and lower the regulator output voltage when raise and lower windings of a reversible electric motor are respectively energized, sensing means for deriving a sample voltage which is proportional to the regulator output voltage, a predetermined reference voltage source, means including a bandwidth setting potentiometer for deriving a lower limit reference potential which is a preselected fraction of said reference voltage, means coupled to said reference voltage source and to the wiper of said bandwidth setting potentiometer for deriving an upper limit reference potential whose magnitude is equal to the sum of said reference voltage plus the difference between said reference voltage and said lower limit reference potential, first and second voltage comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential, a timer adapted to be triggered into operation and to provide an output a preselected time interval after it is started, first and second triggerable semiconductor switching means for respectively energizing said raise and said lower motor windings, means for starting said timer in response to operation of said first voltage comparator means or to operation of said second voltage comparator means, first firing means for triggering said first semiconductor switching means in response to the simultaneous occurrence of outputs from said timer and from said second voltage comparator means, and second firing means for triggering said second semiconductor switching means in response to the simultaneous occurrence of outputs from said timer and from said first voltage comparator means.
 11. In a control for a step voltage regulaTor having means to raise and means to lower the regulator output voltage, in combination, sensing means for deriving a D.C. sample voltage which is proportional to the regulator output voltage, a D.C. reference voltage source, means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than the reference voltage by a fraction of said reference voltage and including bandwidth setting means for selectively adjusting said fraction, first and second voltage comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential, pulse generator means for deriving a train of pulses at the frequency of the power line being regulated, a digital counter receiving said train of pulses from said pulse generator, means normally disabling said counter and being operated to enable said counter in response to an output from said first voltage comparator means or to an output from said second comparator means, digital-to-analog converting means for deriving a D.C. voltage whose magnitude is a function of the count stored in said counter, comparator means for providing a timer output when said D.C. voltage from said converting means rises to the level of a time delay establishing reference potential, means responsive to simultaneous outputs from said timer and said first voltage comparator means for actuating said means to lower the regulator output voltage, and means responsive to simultaneous outputs from said timer and said second voltage comparator means for actuating said means to raise the regulator output voltage.
 12. In a control for a step voltage regulator having means to raise and lower the regulator output voltage when raise and lower windings of a reversible electric motor are respectively energized, sensing means for deriving a D.C. sample voltage which is a function of the regulator output voltage, a D.C. reference voltage source, means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than the reference voltage by a preselected fraction of said reference voltage and including bandwidth setting means for adjusting said preselected fraction, first and second voltage comparator means for comparing said sample voltage respectively to said upper limit reference potential and to said lower limit reference potential, a pulse generator for deriving a train of pulses at the frequency of the power line being regulated, a digital source receiving said train of pulses from said pulse generator, means normally disabling said counter and being operated to enable said counter in response to an output from said first voltage comparator means or to an output from said voltage comparator means, digital-to-analog converting means for deriving a D.C. voltage whose magnitude is a function of the count stored in said counter, comparator means for providing a timer output when said D.C. voltage from said converting means rises to the level of a time delay establishing reference potential, first and second triggerable semiconductor switching means for respectively energizing said raise and lower motor windings, first firing circuit means for triggering said first semiconductor switching means in response to the simultaneous occurrence of an output from said second voltage comparator means and said timer output, and second firing circuit means for triggering said second semiconductor switching means in response to the simultaneous occurrence of said timer output and an output from said first voltage comparator means.
 13. In a control for a step voltage regulator adapted to raise and lower the regulator output voltage when raise and lower windings of a tap changer reversible electric motor are respectively energized, in combination, sensing means for deriving a D.C. sAmple voltage which is a function of the regulator output voltage, a D.C. reference voltage source, means for deriving upper and lower limit reference potentials which are respectively more positive and more negative than said reference voltage by a preselected portion of said reference voltage and including bandwidth setting means for adjusting said preselected portion, first and second voltage comparator means for comparing said sample voltage to said upper limit reference potential and to said lower limit reference potential respectively, a timer, means for starting said timer in response to operation of said first or of said second comparator means, a high frequency oscillator for providing a train of triggering pulses, first and second semiconductor switching means for respectively energizing said raise and lower motor windings, a latch comprising first and second logic gates which are respectively controlled by said first and second comparator means and are cross-connected so that only one logic gate at a time can be enabled, said first and second logic gates each receiving the output of said timer and the triggering pulses from oscillator as inputs and being respectively enabled when the timer, oscillator, and associated comparator means inputs thereto occur simultaneously, and first and second firing means responsive to the enabling of said first and of said second logic gates of said latch respectively for triggering said first and second semiconductor switching means.
 14. In a control in accordance with claim 13 wherein said bandwidth setting means includes a bandwidth setting potentiometer having its resistance element connected in a circuit across said reference voltage source so that said lower limit reference potential is derived at the potentiometer wiper, and said means for deriving an upper limit reference potential includes a unity gain inverter operational amplifier having its inputs respectively coupled to said reference voltage source and to said wiper of said bandwidth setting potentiometer.
 15. In a control in accordance with claim 13 wherein said timer includes a pulse generator for deriving a train of pulses at the frequency of the power line being regulated, a digital counter receiving said train of pulses, digital-to-analog converting means for deriving a D.C. timing voltage whose magnitude is proportional to the count stored in said counter, and comparator means for providing a timer output when said D.C. timing voltage rises to the level of a predetermined time delay establishing reference potential, said means for starting said timer normally disabling said digital counter and being adapted to enable said digital counter in response to operation of said first or of said second voltage comparator means. 